Musnad Abu Ya'la al-Mawsili 244:3

Abu Bakr ibn Abi Shaybah narrated to us, 'Abd al-A'la narrated to us, from Sa'id, from Matar, from Raja', from Qabisah, from 'Amr ibn al-'As, who said: "Do not confuse for us the Sunnah of our Prophet, may Allah bless him and grant him peace. The waiting period ('iddah) of the umm al-walad (a slave woman who has borne a child to her master) is the waiting period of the woman whose husband has died."